1. Field of the Invention
The invention relates to the drilling of wells offshore; and, more particularly, to a method and apparatus for producing hydrocarbons from subterranean formations under the ocean floor.
2. Description of the Prior Art
In an attempt to locate new oil fields, an increasing amount of well drilling has been conducted at offshore locations, such, for example, as off the coasts of California, Louisiana and Texas, and, more recently, off the coast of Alaska and in the North Sea. Generally, platform structures are installed on the ocean floor and straight well conductors are extended from the platform structure some distance below the ocean floor, providing guidance and support for wells drilled into hydrocarbon-bearing reservoirs.
In order to increase the lateral reach of such well conductors, these conductors have been slanted from the vertical, as for example, in a U.S. Pat. No. 3,451,493 to Storm. Such conductors are usually extended laterally beyond the perimeter of a conventional platform structure at a considerable distance above the mudline (e.g., about 100 to 150 feet in 250-foot water depth), This condition is undesirable since it results in long spans of well conductors which are unsupported or which require fabrication of conductor guides outside of the main framework of the platform structure. Such an arrangement is both costly and complicated. Further, if such well conductors are to perform as a portion of the platform structure foundation by providing resistance to lateral movement, the cost of the outrigged framework is considerably increased.
An alternate type of arrangement in applying such straight-slant type conductors to deep water conditions is to keep the portion of the conductor which is above the mudline inside the framework of the platform by crisscrossing them in a vertical plane. The wellhead for each well conductor is located at the top of the platform structure on one side thereof and the bottom well conductor platform support is located at the mudline near the opposite side of the platform structure. Such an arrangement requires that a large part of the deck area of the platform structure be designed to accommodate the required slant drilling rig since the wellheads are not located together. Unless the layout of the platform structure is axisymmetric, this latter technique may result in an undesirable well conductor reach pattern. Moreover, a special slant drilling rig must be used rather than the conventional straight rig.
Thus, the disadvantages of such prior art arrangements for increasing horizontal reach are conductor guides external of the platform structure, possible interference of conductors with the platform structure foundation, non-centrally located wellheads, possibly undesirable well conductor reach patterns, and the need for a special rig. These disadvantages become especially significant in water deeper than 200 feet.